A conventional method for resistance welding between the cells of in a lead acid battery carries out the welding by means of a constant current control system or a constant voltage control system. More specifically, it has been general that lugs of mutually opposite polarities projecting upwards from the respective ends of straps connecting respective groups of tabs of cell assemblies incorporated in the respective cell chambers of a lead acid battery are so positioned as to face each other through a hole made in a partition wall partioning the adjacent cell chambers, and a pair of resistance welding guns, i.e., electrodes of a resistance welding machine are applied to the outer surfaces of the lugs of both the straps and are pressed to cause both lugs to become attached under pressure to both the surfaces of the partition wall and further cause the central portions of both the lugs to become dented and thrust into the hole by the projections of the guns, and in this condition, a current is applied thereto through the guns by the constant current control system or the constant voltage control system for performing a resistance welding between both the lugs, i.e., the so-called cell-to-cell resistance welding.
The foregoing welding method of the conventional constant current control system or the constant voltage control system is subjected to the following problems, however. Namely, a contact area in the central portion of each of the two lugs in which they make contact with each other under pressure is not consistent due to differences in hardness of the lugs of the straps, changes in the positions of the projections when the outer surfaces of the mutually facing lugs are pressed with the pair of guns (electrodes) having the projections, differences in thickness of the partition walls, variable diameters of the holes, etc. Therefore, a quantity of heat generated with the current application thereto by the conventional constant current control system or constant voltage control system is not unvarying as clearly indicated in the following. Namely, in the case of the constant current control system, the current I is kept constant, so that when the contact area in which the two electrodes have contact with each other is comparatively small, an electric resistance value R becomes large, thus resulting in a larger quantity of heat Q=kI.sub.2 RT. Inversely, if the contact area is comparatively large, the electric resistance value R becomes small, and accordingly the quantity of heat becomes small.
On the other hand, in the constant voltage control system an electric voltage V=IR is constant, so that if the contact area between the two lugs is comparatively small, the resistance value becomes large, and accordingly the current becomes so much smaller, and as a result, the quantity of heat Q=kI.multidot.IRT becomes small. Inversely, if the constant area between the two lugs is large, the resistance value R becomes small, and accordingly the electric current value I becomes large, and as a result, the quantity of heat becomes large.
Thus, the contact area formed when the two lugs are pressed for their contact with each other by the welding guns is varied due to the foregoing various reasons and the generated quantity of heat fluctuates accordingly, so that it is not possible to obtain a stable welding through the constant current control system or the constant voltage control system and, in some occasions, an imperfect welding is more liable to result therefrom. In addition, with the constant current control system or the stop voltage control system, there are involved the following problems when examined as to the course of the current application to the lugs during the resistance welding. Namely, in the constant current control system, if the mutual contact area is small at the beginning of current application for welding, sparks are liable to occur and in the course of welding by the applied current, the contact area becomes larger, but it is such that when there is developed a large void, the current is kept constant notwithstanding, that is, the current is not lowered and thus there fly up sparks, at which time molten lead is scattered and attached to the portion between a negative plate and a positive plate of the cell assembly. This is liable to cause short-circuits while resulting in a defective weld. As for the constant voltage control system, if the contact area is small, sparks occur at the beginning of the current application for welding, and in the course of welding the contact area becomes larger to increase the current accordingly, and when the voids are formed, sparks are generated to cause scattering of the molten lead which in turn is liable to cause the short-circuits in a similar fashion as mentioned above while incurring a defective weld.